Process for increasing the selectivity of the hydrogenation of 4,4′-diaminodiphenylmethane to 4,4′-diaminodicyclohexylmethane in the presence of an N-alkyl-4,4′-diaminodiphenylmethane

ABSTRACT

The invention relates to a process for increasing the selectivity of the hydrogenation of 4,4′-diaminodiphenylmethane (4,4′-MDA) to diaminodicyclohexylmethane (4,4′-HMDA) by catalytic hydrogenation of a mixture containing 4,4′-MDA as the main component and its mono-N-methyl derivative as a secondary component. According to the invention, the hydrogenation is terminated before a conversion of 4,4′-MDA to 4,4′-HMDA of 99% is achieved. Under these conditions, a substantially smaller proportion of the N-methyl-4,4′-MDA is hydrogenated to N-methyl-4,4-HMDA.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national stage application of International PatentApplication No. PCT/EP2003/006669, filed on Jun. 25, 2003, and claimspriority to German Patent Application No. 102 31 119.6, filed on Jul.10, 2002, both of which are incorporated herein by reference in theirentireties.

The invention relates to a process for increasing the selectivity of thehydrogenation of 4,4′-diaminodiphenylmethane to4,4′-diaminodicyclohexylmethane from a mixture of substances which, inaddition to 4,4′-diaminodiphenylmethane and optionally its 2,4′- and2,2′-isomers, also contains N-alkyl derivatives of these substances,particularly N-methyl-4,4′-diaminodiphenylmethane.

4,4′-Diaminodicyclohexylmethane, also known as hydrogenatedmethylenedianiline or abbreviated as 4,4′-HMDA, is an important buildingblock for the production of isocyanates, which are used in variousresins and paints, and a hardener for epoxy resins. 4,4′-HMDA occurs asthree configurational isomers, i.e. as trans-trans-, trans-cis andcis-cis. For the field of application in resins and paints, a 4,4′-HMDAwith a low content of trans-trans isomers is desired; this content isgenerally less than 30%, preferably in the range of 15 to about 25%.

4,4′-Diaminodicyclohexylmethane (4,4′-HMDA) can be obtained by a methodthat is known per se by the catalytic hydrogenation of4,4′-diaminodiphenylmethane (=4,4′-methylenedianiline=4,4′-MDA). Inindustrial practice, not pure 4,4′-MDA but a reaction mixture from theproduction of 4,4′-MDA is used, which additionally contains the isomers2,4′-MDA and 2,2′-MDA as main components. As a result of theirproduction, these reaction mixtures can also contain additionalamino-group-containing polynuclear compounds. In addition, they cancontain various N-alkylated compounds of the corresponding aromaticdiamines, particularly mono-N-methyl-4,4′-diaminodiphenylmethane incontents of generally 0.05–2%, usually 0.1 to 1%.

The catalytic hydrogenation of 4,4′-MDA to 4,4′-HMDA with hydrogen on aruthenium-containing, heterogeneous catalyst has been described inseveral patent applications. In a first version, for example, theequilibrium composition of the mixture of isomers consisting of thethree HMDA isomers, containing about 50–55% of the trans-trans isomer,is produced as a result of the selection of the hydrogenationconditions. The product is also known by the term PACM-50. Thedescription of this version can be taken e.g. from U.S. Pat. No.3,766,272.

Since the separation of the three stereoisomers of HMDA is technicallyvery complex and burdened with high costs, another version is aimed atthe direct production of a product mixture with a proportion of 15–25%of the trans-trans isomer, so-called PACM-20. Numerous versions of thisprocess variant are known, which differ primarily in the selection ofthe catalyst, support material and reaction conditions. Thus, the EPpatent 0 324 190 describes that the hydrogenation can be performed at 50to 350 bar and 100 to 190° C. if the supported catalyst has-a BETsurface area in the range of 70 to 280 m²/g and an average pore diameterd_(p) of 10 to 320 Å, the catalyst contains 0.1 to 5 wt. % ruthenium andthe depth of penetration is at least 50 μm.

The U.S. Pat. No. 4,394,523 sets forth a generic process for theproduction of 4,4′-MDA with a low content of trans-trans isomer, whereinRu on aluminium oxide is used as the supported catalyst and thehydrogenation is performed under an H₂ pressure of at least 36.5 bar inthe presence of an aliphatic alcohol and ammonia.

In the generic process according to EP-A 0 231 788, a supported catalystcontaining rhodium and ruthenium is used. In the process according toDE-OS 40 28 270, the Ru supported catalyst preferably contains 1 wt. %Ru and additionally compounds of rare earth metals and manganese, andthe preferred hydrogen pressure required is in the range of 200 to 400bar. Finally, EP-A 0 639 403 teaches a version in which the Ru supportedcatalyst is characterised by a special clay support and thehydrogenation takes place according to the examples under a high H₂pressure of 300 bar. When this process was reproduced undersubstantially lower hydrogen pressure, it was shown that this onlyenabled 4,4′-HMDA to be obtained in an inadequate yield.

Consequently, while it is true that the hydrogenation of 4,4′-MDA is awell-known process, a particular problem occurs when, instead of pure4,4′-MDA, a technical mixture of substances containing this as the maincomponent, such as that resulting from the production of MDA fromaniline and formaldehyde, is hydrogenated. Particularly critical is thequantity of mono- or polyalkylated aromatic diamines present, includingparticularly mono-N-methyl-4,4′-MDA.

Mono-N-methyl-4,4′-MDA is converted to the corresponding N-methylatedcycloaliphatic diamine by the hydrogenation process in the same way as4,4′-MDA. This compound, owing to the physical properties that aresimilar to the HMDA isomers, can only be separated from the productmixture using very complex apparatus and with a large loss of 4,4′-HMDAyield. This N-methylated cycloaliphatic diamine leads to problems, evenin the trace range, if the corresponding diisocyanate is to be producedfrom the diamine in subsequent processing, e.g. by phosgenation.

Against this background, a version of hydrogenation is desirable inwhich the non-N-alkylated compounds are preferably selectivelyhydrogenated from a mixture of substances containing the MDA isomers andN-alkyl MDA.

The U.S. Pat. No. 5,360,934 describes a process in which thecorresponding cycloaliphatic amines are produced from mixtures ofsubstances containing various mono- or binuclear aromatic amines,N-alkylated aromatic amines and such compounds. However, no way ofhydrogenating only the non-N-alkylated component with high selectivityfrom a mixture of substances is set forth.

In the process according to EP-A 0,392,435, the hydrogenation of an MDAcrude product is described, which can contain oligomers and formamidederivatives of MDA. The hydrogenation is performed in a batch autoclaveto complete conversion (99–100%). There is no mention of any influenceof the way in which the process is conducted on a selective inhibitionof the hydrogenation of N-alkylated compounds of MDA.

In EP 0 355 272 A, the Rh-catalysed hydrogenation of an MDA crudeproduct containing alkyl-substituted aromatic amines is mentioned. It isdisclosed that such compounds do not have a negative effect on thehydrogenation of the main component and do not poison the catalyst.There are no references to a selective hydrogenation of thenon-N-alkylated aromatic amines.

EP 0 231 788 A teaches a process for the hydrogenation of MDA on acatalyst containing Rh and Ru. In the tests 29–34, a crude MDAcontaining 0.3% N-methyl-4,4′-MDA is hydrogenated. With full conversion,a product with only a small proportion of high boilers is achieved. Thehydrogenation of the N-methyl-4,4′-MDA contained in the crude MDA andthe solution of the selectivity problem on which the present inventionis based are not dealt with.

Finally, EP 0 001 425-A also teaches the hydrogenation of a crudeproduct containing 4,4′-MDA. It is mentioned that binuclear aromaticsare hydrogenated more readily than trinuclear ones, but thehydrogenation problem of the present invention is not addressed.

The object of the present invention is therefore to set forth a processfor the catalytic hydrogenation of 4,4′-MDA to 4,4′-HMDA, which leads tothe highest possible yield of resulting cycloaliphatic diamines with thehighest possible conversion of the aromatic diamines, wherein thehydrogenation product should have the smallest possible proportion ofmono- or poly-N-alkyl-substituted cycloaliphatic diamines.

A further object is aimed at modifying the hydrogenation conditions ofthe process for the hydrogenation of crude MDA which is known per se insuch a way that the hydrogenation of N-alkyl derivatives in the crudeMDA is at least partially inhibited and thus the selectivity with regardto 4,4′-HMDA is increased.

EP 1 251 119 A1, which is a document according to article 54(3) EPC,pertains to a continuous process for the preparation ofdiaminodicyclohexylmethane (PACM). It deals with a suspensionhydrogenation, wherein MDA is hydrogenated to a conversion of at least95%, preferably at least 99%. The problem underlying the presentapplication, namely inhibiting the hydrogenation of N-methylatedby-products contained in the crude MDA, was not addressed. Accordingly,no technical teaching for solving the problem was disclosed. Thepreferred embodiment—MDA conversion of at least 99%—given in thisdocument rather leads away from the problem solution of the presentapplication.

A process has been found for the production of4,4′-diaminodicyclohexylmethane (4,4′-HMDA) by catalytic hydrogenationof a mixture of substances containing 4,4′-diaminodiphenylmethane(4,4′-MDA) as the main component and its mono-N-methyl derivative as asecondary component with increased selectivity with regard to thehydrogenation of 4,4′-MDA in the presence of a heterogeneoushydrogenation catalyst at a temperature in the range of 50 to 220° C.and a hydrogen pressure in the range of 1 to 30 MPa, characterised inthat the hydrogenation is terminated before a conversion of 4,4′-MDA to4,4′-HMDA of 99% is reached.

The process according to the invention is suitable not only for thecatalytic hydrogenation of substantially pure4,4′-diaminodiphenylmethane (4,4′-MDA) with a small proportion ofmono-N-methyl-4,4′-diaminodiphenylmethane, but also for the catalytichydrogenation of reaction mixtures from the production of 4,4′-MDAwhich, in addition to 4,4′-MDA as the main component, also contain oneor more isomeric compounds from the series 2,4′-diaminodiphenylmethaneand 2,2′-diaminodiphenylmethane and, resulting from the production,possibly other polynuclear compounds, together with mono- orpoly-N-alkylated compounds of the diaminodiphenylmethanes.

A particular advantage of the present invention lies in the fact that,to achieve the objects set, compared with the prior art, neither aparticular catalyst nor particular additives, such as ammonia or lyes,are necessary. Surprisingly, the hydrogenation of the N-alkyl-MDAs issuccessfully inhibited, at least partially, by controlling theconversion of 4,41-MDA so that 4,4′-HMDA is obtainable with a reducedcontent of N-alkyl-HMDA.

According to a preferred embodiment, the hydrogenation is carried out upto a 4,4′-MDA conversion in the range of about 90 to 98.9, particularly95 to 98%.

According to a preferred embodiment, those catalysts are selected andthe hydrogenation is carried out under those conditions that enable4,4′-HMDA to be obtained with a trans-trans proportion in the range of15 to 25%, particularly 18 to 23%. Examples of features can be takenfrom the following documents:

-   -   EP 0 639 403 A2, EP 0 814 098 A2, DE 199 42 813, EP patent        application 02 012 040.8, EP 0 873 300 B1, EP 0 066 211 A1, EP 0        324 190 B1.

The hydrogenation takes place at a temperature in the range of 50 toabout 220° C., particularly 70 to 190° C. and preferably at 90 to 150°C. The H₂ pressure is generally in the range of 1 to 30 MPa, and thepressure particularly amounts to at least 3 MPa. With an appropriateselection of catalyst, particularly Ru supported catalysts with a BETsupport surface area of less than 70 m²/g, it is possible to hydrogenateunder an H₂-pressure in the range of 3 to 15 MPa, particularly 5 to 10MPa.

In relation to the selection of catalyst, reference is made to the priorart already cited. Supported catalysts with ruthenium or rhodium or aRu/Rh combination as essential active metals and a support material fromthe series of e.g. activated carbon, inorganic oxides, such as inparticular Al₂O₃, SiO₂, TiO₂, ZrO₂, ZnO and MgO, and also bentonite,aluminosilicates, kaolins, clays, kieselguhr and diatomaceous earth arepreferred.

As is known from the prior art, the specific surface area, the poredistribution and the ratio of the surface area of the active metal tothe surface area of the support influence the efficiency of thecatalyst, i.e. its activity at different pressures and temperatures, itslifetime and selectivity with regard to the stereoisomers of 4,4′-HMDA.Oxidic supported catalysts with a BET surface area in the range of >30to <70 m²/g, when more than 50% of the pore volume is formed frommacropores (>50 nm) and less than 50% from mesopores (2–50 nm), areparticularly preferred. Supports with a BET surface area of less than 30m², particularly 0.5 to 10 m²/g, are also highly suitable. The Rucontent of the catalyst is generally in the range of 0.1 to 20 wt. %,preferably 0.5 to 10 wt. %.

An embodiment in which the mixture of substances containing MDA andN-methyl-MDA is hydrogenated in a solvent that, for its part, does notas far as possible have an alkylating effect on the amines formed andcontained in the reaction mixture, is also preferred. A solvent ispreferably used in a quantity of about 10 to 90 wt. %, based on thesolution of the aromatic amine to be hydrogenated.

Suitable solvents are e.g. primary, secondary and tertiary mono- orpolyhydric alcohols, such as methanol, ethanol, n- and i-propanol, n-,sec.- and tert.-butanol, ethylene glycol, ethylene glycolmono(C₁–C₃)alkyl ether; cyclic ethers, such as tetrahydrofuran anddioxane; alkanes, such as n- and iso-alkanes with 4–12 C atoms, e.g.n-pentane, n-hexane and isooctane, and cyclic alkanes, such ascyclohexane and decalin. Whereas alcohols can in some cases have analkylating effect, ethers do not exhibit these disadvantages.

A preferred solvent is tetrahydrofuran. Particularly preferably, areaction mixture in which the crude substance mixture is present in aconcentration of 5–30% in tetrahydrofuran is hydrogenated.

The hydrogenation can also be carried out in the presence of ammonia ora primary, secondary or tertiary amine or a polycyclic amine with abridged N atom. It is usefully ensured by preliminary tests that noundesirable alkylation and/or isomerisation of the 4,4′-HMDA in thedirection of a higher trans-trans proportion takes place under theconditions selected.

For continuous hydrogenation, a fixed bed reactor is preferred. Thefixed bed reactor can be operated as a bubble column reactor, but atrickle-bed method is preferred. A trickle-bed reactor is preferablyused and operated with an LHSV value in the range of 0.1 to 5 h⁻¹(=litres of the reaction solution of the aromatic amine to behydrogenated per litre of fixed bed catalyst per hour). According to aparticularly preferred embodiment of the process according to theinvention, a tube bundle reactor is used and this is operated by atrickle-bed method.

By controlling the conversion of 4,4′-MDA to 4,4′-HMDA according to theinvention, i.e. terminating the hydrogenation before a conversion of 99%is achieved, it has become possible to minimise the proportion ofN-alkylated derivatives of 4,4′-HMDA and its 2,4′- and 2,2′-isomers. Byreducing the 4,4′-MDA conversion by a few percent, the content ofN-methyl-4,4′-HMDA in the hydrogenated reaction mixture is successfullyreduced by a multiple compared with the content of N-methyl-4,4′-MDAcontained in the crude MDA.

EXAMPLES

The hydrogenation was performed continuously in a trickle-bed plantconsisting of three reactors connected in series, each with a reactorcapacity of 2500 ml. The plant consisted of a liquid feed, the reactorsand a liquid separator. The reaction temperature was adjusted for eachreactor separately using heat transfer medium-oil circulations. Thepressure and hydrogen flow were regulated electronically. The solutionof the MDA crude mixture in tetrahydrofuran (THF) or in methanol (MeOH),which additionally contained 1% ammonia in the case of methanol assolvent, was metered into the hydrogen stream with a pump and themixture fed into the top of the first reactor (trickle-bed method). Fromthere it passed through all three reactors in the same way. After thesolution had trickled through the reactors, a sample was taken atregular intervals after each reactor. A separate sampling point wasprovided after each reactor for this purpose.

The crude MDA used was available in two different grades.

Grade A contained 78 wt. % 4,4′-MDA, 11 wt. % 2,4′-MDA, 0.8 wt. %2,2′-MDA, 9 wt. % polynuclear high boilers and 0.19 wt. %N-methyl-4,4′-MDA.

Grade B contained 97.5 wt. % 4,4′-MDA, 1.7 wt. % 2,4′-MDA and 0.75 wt. %N-methyl-4,4′-MDA.

In example 7 (B 7) and the comparative examples (VB 5–7) the reactionsolution was recycled into the reactor several times to determine theconcentration profiles of the reactants at higher conversions.

The Ru supported catalysts H 2017 H/D (catalyst A) and B 4245 (catalystB) from Degussa were used, both 5% ruthenium on aluminium oxide supportextrudates with a diameter of 1.1–1.3 mm.

The results of the catalytic hydrogenation of the mixture of 4,4′-MDAand N-methyl-4,4′-MDA to the corresponding cycloaliphatic compounds4,4′-HMDA and N-methyl-4,4′-HMDA using the catalysts mentioned above canbe taken from the table.

The conversion given in the table refers to the conversion of the4,4′-MDA, 2,4′-MDA and 2,2′-MDA and the intermediates 4,4′-, 2,4′-,2,2′-diaminocyclohexylphenylmethane participating in the reaction to4,4′-, 2,4′-, 2,2′-HMDA. Accordingly, a 100% conversion means thehydrogenation of all aromatic double bonds of the three MDA isomers. Inaddition, the sole conversion of 4,4′-MDA to 4,4′-HMDA is included inthe table by analogy. However, no significant difference is apparentcompared with the conversion value for all three MDA isomers.

TABLE Sampling after proportion Example (B) Crude ConcentrationTemperature Conversion (%) of the Comparative MDA of crude (° C.) (2,2′,2,4′ and 4,4 Conversion (%) N-Methyl- total example VB) Catalyst gradeSolvent MDA [wt. %] Reactor 1/2/3 isomer) (4,4 isomer only) 4,4′-HMDA(%) reaction path VB 1 A A THF 12.5  95/100/105 99.5 99.6 0.18 3/3 B 1 AA THF 12.5  90/95/100 98.4 98.5 0.15 3/3 B 2 A A THF 12.5  95/100/10095.7 95.8 0.14 2/3 B 3 A A THF 12.5  90/95/100 90.8 91.0 0.12 2/3 VB 2 BA THF 12.5  95/102/105 99.9 99.9 0.19 3/3 B 4 B A THF 12.5  95/100/10597.4 97.6 0.15 3/3 B 5 B A THF 12.5 100/105/110 95.0 95.4 0.13 3/3 VB 3B B THF 12.5  83/98/105 98.8 98.8 0.67 3/3 VB 4 B B THF 12.5  83/98/10599.6 99.6 0.69 3/3 B 6 B B THF 12.5  83/98/105 95.4 95.4 0.49 2/3 VB 5 AA MeOH 26 105/105/105 99.7 99.8 0.38 11/12 VB 6 A A MeOH 26 105/105/10599.5 99.5 0.21 10/12 VB 7 A A MeOH 26 105/105/105 99.0 99.0 0.21  8/12 B7 A A MeOH 26 105/105/105 97.0 97.0 0.16  6/12

The table shows that a 1.3% reduction in the conversion of MDA to HMDA(from 99.5% in VB1 to 98.4% in B1) brings about more than aproportionally large reduction in the formation of N-methyl-4,4′-HMDA of16.6% (from 0.18% in VB1 to 0.15% in B1).

The effect according to the invention becomes even clearer whencomparing VB 3 and VB 4 with B6. Whereas in VB3, with a conversion of98.8%, almost all the N-methyl-MDA present was hydrogenated, and evenwhen increasing the MDA conversion to 99.6% (VB 4) no further changecould be observed, in B6 with a conversion of 95.4%, only 72% of theN-methyl-MDA was hydrogenated.

The process can even be applied when using a solvent that itselfalkylates in traces. VB 6 and VB 7 show here too that, with conversionsof >99%, all the N-methyl-MDA is hydrogenated, whereas in B 7 with aconversion of 97.0% only 76% is hydrogenated. If, as in VB 5, theconversion is increased beyond this, a further alkylation of HMDA by themethanol present becomes apparent, even with the simultaneous presenceof NH₃.

The effect according to the invention can be demonstrated for bothcatalysts A and B in a similar way. The effect is particularlysignificant if the conversion of 4,4-MDA′ to 4,4′-HMDA is kept within arange of 90% to less than 99%.

1. A process for the production of 4,4′-diaminodicyclohexylmethane(4,4′-HMDA) by catalytic hydrogenation of a mixture comprising4,4′-diaminodiphenylmethane (4,4′-MDA) as the main component and amono-N-methyl derivative thereof as a secondary component with increasedselectivity with regard to the hydrogenation of 4,4′-MDA in the presenceof a heterogeneous hydrogenation catalyst at a temperature in the rangeof 50 to 220° C. and a hydrogen pressure in the range of 1 to 30 MPa,wherein the hydrogenation is terminated before a conversion of 4,4′-MDAto 4,4′-HMDA of 99% is achieved.
 2. The process according to claim 1,wherein a crude MDA, comprising at least 70 wt. %4,4′-diaminodiphenylmethane and 0.01 to 2 wt. %N-methyl-4,4′-diaminodiphenylmethane, is used as said mixture.
 3. Theprocess according to claim 2, wherein said mixture comprises 75–99 wt. %4,4′-MDA, 1–11 wt. % 2,4′-MDA, less than 2 wt. % 2,2′-MDA and up to 1wt. % N-methyl-4,4′-MDA.
 4. The process according to claim 1, whereinthe hydrogenation of 4,4′-diaminodiphenylmethane to4,4′-diaminodicyclohexylmethane is terminated at a conversion in therange of 90% to 98.9%.
 5. The process according to claim 1, wherein thehydrogenation is performed at a temperature in the range of 90 to 150°C. and a pressure in the range of 5 to 15 MPa.
 6. The process accordingto claim 1, wherein an Ru-supported catalyst with an Ru content of 0.5to 10 wt. % is used.
 7. The process according to claim 6, wherein anRu-aluminium oxide or Ru-titanium dioxide supported catalyst is used asthe Ru supported catalyst, the support having a BET surface area of lessthan 70 m²/g.
 8. The process according to claim 1, wherein the catalytichydrogenation is performed in the presence of a solvent from the seriesof the ethers.
 9. The process according to claim 1, wherein thecatalytic hydrogenation is performed in a continuous operating method ina fixed bed reactor packed with an Ru supported catalyst, wherein thereactor is operated by a trickle-bed method.
 10. The process accordingto claim 8, wherein the catalytic hydrogenation is performed in acontinuous operating method in a fixed bed reactor packed with an Rusupported catalyst, wherein the reactor is operated by a trickle-bedmethod.